1. Field of the Invention
The present invention relates to electronic components and more particularly to providing compensation for environmental effects on operation of oscillators.
2. Description of Related Art
Crystal oscillators, which are components in electronic systems that are widely used to generate fixed frequency signals, are known to be affected by environmental conditions, such as temperature. In general, the frequency generated by a crystal oscillator may vary as the temperature around it changes. The applications in which such crystal oscillators are used may not tolerate much variation in the frequency. Several methods have been developed to stabilize their frequency. Some examples include:                1. Having the crystal oscillator operate in a controlled temperature environment. Such crystal oscillators are generally known as an oven controlled crystal oscillator (OCXO).        2. Combining the crystal oscillator circuit with analog circuits, which include components that have temperature-sensitive reactances and the components are arranged to compensate the frequency vs. temperature characteristic. Such crystal oscillators are called temperature compensated crystal oscillators (TCXO).        3. Using crystal oscillators that have values of frequency vs temperature characteristics stored in digital memory. The values may be stored and used as an interpolated look-up table by a specialized hardware circuit to correct the output frequency of the oscillator. In order to know which value to access in the look-up table, an analog to digital converter (A/D) is used to measure the value of a temperature sensor. This is also known as a microprocessor compensated crystal oscillator (MCXO) or Digitally Compensated Crystal Oscillator (DCXO.)        4. Storing coefficients, such as polynomial coefficients, that are used to generate curves that match the frequency vs temperature characteristic, in a digital memory inside the crystal oscillator. In this case, the functional nature of the curves is predefined. For example, a correction equation Correction=k*T^3 may be used. That is, some constant coefficient k times the cube of the temperature, and the coefficient k is calculated and stored by the manufacturer in digital memory. As in the MCXO above, a temperature sensor is digitized and used with the stored coefficient to calculate a frequency correction which is applied to specialized hardware to correct the frequency. Usually several coefficients are used for a realistic curve fit.        5. Using a crystal that vibrates in two modes at the same time, one mode being relatively stable and the other mode being highly temperature-sensitive, such that it serves as a temperature sensor by measuring the oscillation frequency with moderate precision relative to the first oscillation mode. Then the difference between the two frequencies may be used to generate a correction factor.        6. Using two crystals in one package, where one crystal being the unit to be compensated, and the other crystal being one having a linear frequency versus temperature characteristic so that the second crystal acts as a temperature sensor for the first crystal. In this case, the frequency of the sensor can provide temperature information which may then be used to correct the first crystal frequency if the correction required is previously measured and known. This is similar to no. 5 above, except the use of two crystals isolate some problems of frequency cross-coupling in a dual-mode device.        
The above-described methods rely on a static curve model for temperature dependence. This means the models assume there is a frequency vs. temperature relation that does not change for a given crystal unit, and does not depend on how fast the temperature is changing. It is well known in the art that this assumption begins to fail with increasing error when the temperature rate of change increases from zero.
There is a need for improved systems and methods for stabilizing crystal oscillator operation.